Host devices such as computers, laptops, personal video recorders (PVRs), MP3 players, game consoles, servers, set-top boxes, digital cameras, and/or other electronic devices often need to store a large amount of data. Storage devices such as hard disk drives (HDD) may be used to meet these storage requirements.
Referring now to FIG. 1, an exemplary hard disk drive (HDD) 10 is shown to include a hard disk drive (HDD) 12 and a hard drive assembly (HDA) 13. The HDA 13 includes one or more hard drive platters 14 that are collectively called a spindle. The platters 14 are coated with magnetic layers 15. The magnetic layers 15 store positive and negative magnetic fields that represent binary 1's and 0's. A spindle motor, shown schematically at 16, rotates the hard drive platters 14. Generally, the spindle motor 16 rotates the hard drive platters 14 at a fixed speed during read/write operations. One or more read/write actuator arms 18 moves relative to the hard drive platters 14 to read and/or write data to/from the hard drive platters 14.
A read/write device 20 is located near a distal end of the read/write arm 18. The read/write device 20 includes a write element such as an inductor that generates a magnetic field. The read/write device 20 also includes a read element (such as a magneto-resistive (MR) element) that senses the magnetic field on the platter 14. A preamp circuit 22 amplifies analog read/write signals.
When reading data, the preamp circuit 22 amplifies low level signals from the read element and outputs the amplified signal to a read/write channel device 24. When writing data, a write current is generated which flows through the write element of the read/write device 20. The write current is switched to produce a magnetic field having a positive or negative polarity. The positive or negative polarity is stored by the hard drive platter 14 and is used to represent data.
The HDD 12 typically includes a buffer 32 that stores data that is associated with the control of the hard disk drive and/or buffers data to allow data to be collected and transmitted as larger data blocks to improve efficiency. The buffer 32 may employ DRAM, SDRAM or other types of low latency memory. The HDD 12 further includes a processor 34 that performs processing that is related to the operation of the HDD 10.
The HDD 12 further includes a hard disk controller (HDC) 36 that communicates with a host device via an input/output (I/O) interface 38. The I/O interface 38 can be a serial or parallel interface, such as an Integrated Drive Electronics (IDE), Advanced Technology Attachment (ATA), or serial ATA (SATA) interface. The I/O interface 38 communicates with an I/O interface 44 that is associated with a host device 46.
The HDC 36 also communicates with a spindle/voice coil motor (VCM) driver 40 and/or the read/write channel device 24. The spindle/VCM driver 40 controls the spindle motor 16 that rotates the platters 14. The spindle/VCM driver 40 also generates control signals that position the read/write arm 18, for example using a voice coil actuator, a stepper motor or any other suitable actuator.
Referring now to FIG. 2, data is typically written on the platters 14 in concentric circles called tracks 50. The tracks 50 are divided radially into multiple sectors 52. As the diameter of the tracks 50 decreases toward the center of the platter 14, the sector size decreases. Before performing a read or a write operation on a sector of a track, a head locks onto the track by referring to positioning information called servo that is generally prewritten on the platters. The servo provides the positioning information so that the heads know where to write data on the platters 14 during a write operation and where to read data from during a read operation.
Traditionally, the servo is prewritten in multiple sectors using a special servo writing apparatus when a disk drive is manufactured. The traditional servo writing methods, however, become impractical as the track density, that is, the number of tracks per inch, increases for a disk drive. More recently, the track density has increased as the demand for storage capacity and spin rates of disk drives is increasing. Additionally, the diameter of disk platters is shrinking so that the drives can fit into smaller devices such as palmtops and other handheld devices that require disk drives that are small in physical size and high in storage capacity.
The increasing track density also makes traditional servo writing physically impractical. Accordingly, modern disk drives increasingly use self-servo-write (SSW) methods to write their own servo sectors using the same read/write heads that are used to read/write regular data. When writing the servo using the SSW methods, the heads typically lock onto reference servo sectors (RSS) that are prewritten on the platters in the form of spirals.
The spirals, however, are sometimes written imperfectly. Moreover, when reading the spirals, the spindle speed may fluctuate slightly. Additionally, the actuator arm may not be perfectly steady when locked onto an RSS. Consequently, a head may not be able to quickly and accurately lock onto an RSS as the head moves across a platter.